1. Field of the Invention
The present invention relates generally to a fiber optic microbend sensor, and in particular, it is directed to a braided fiber optic microbend sensor for the measurement of displacement, strain, or elongation.
2. Description of the Related Art
Extensometers are devices used to measure displacement or length changes of structural members or test articles due to mechanical or thermal loads. Typically, an extensometer is attached to the two points between which the change in length is expected. The physical changes in length are then converted to an electronic, optical, or magnetic signal proportional to the length change.
Commercially available extensometers may be based on measurement of differential capacitance, motion of a magnetic core such as an LVDT or DCDT, measurement of strain, rotation of a potentiometer, and other means to achieve the desired sensitivity over the required gage length. An apparatus and method for performing extensometry using fiber optics is a microbend sensor, as shown in FIG. 1. In this sensor, the optical fiber 10 (consisting of a core, cladding and buffer coating) is clamped between two sets of opposing corrugations 12, 14. Changes in the length of the structure (not shown) to which the corrugated members 12, 14 are attached changes the separation of the corrugations and thereby bending of the optical fiber between the corrugations. Light is propagated through the core of the fiber; some of the light is lost from the core to the cladding through mode conversion as a result of the microbending between the corrugations, causing change in intensity of the light exiting the fiber 10. This change in intensity can be readily related to the change in separation of the corrugations, i.e., intensity change is proportional to the change in separation.
The technical paper, "Fiber Optic Pressure Sensor", J. N. Fields, C. K. Asawa, O. G. Raimer and M. K. Barnowski, J. Acoust. Soc. Am. 67(3), March, 1980, at pages 816-818, teaches that is known to apply a succession of axial bends to an optical fiber by means of a pair of corrugated plates to attenuate the intensity of light transmitted along the fiber. Likewise, U.S. Pat. Nos. 4,421,979; 4,459,477; 4,477,725; and 4,463,254 all issued to Asawa, et al teach of the application of microbend force transducers using the principle illustrated in FIG. 1 applied to long structures such as oil and gas pipeline and the like.
In a similar fashion, U.S. Pat. No. 4,568,408 to Schmadel, et al relates to a fiber optic energy sensor and optical demodulation system. It teaches that if a single mode optical fiber is compressed radially, or stretched, or compressed longitudinally, then the optical path length for electro-magnetic radiation traveling in the core of a single mode optical fiber changes.
Microbend sensors in general offer the advantages of being simple, low cost, immune to electro-magnetic interference, operable over wide ranges of temperature, pressure and other environmental conditions. In addition, good performance can be obtained with relatively simple opto-electronics.
In the past fiber optic microbend technology required additional external structures such as corrugated plates. These additional external structures add cost in terms of material and time. There is a need for a fiber optic microbend sensor which is simple, accurate, and readily attaches to structures, that does not require precision in the attachment and is low cost. Additionally, this fiber optic microbend sensor must provide all the advantages of fiber optic microbend sensors over conventional electromagnetic sensors.